agars

Hi
I am having trouble with my experiment (water analysis).
The method just states "agar" as a solidifying agent and
we bought Agar Grade A. Somehow, it won't set.
The author of the method recommended Bacto Agar.
Is there any difference between these two agars?
If there are so many solidifying agars (which are not selective) then how do
you go about choosing the right one for your experiment?
Kei